It fights teachers for their kids grade, parks its mini van in spots marked compact, sues a business when it trips over Caution tape, sits at an AYCE buffet for lunch and dinner, etc. In a nutshell, there are no rules of etiquette. There are only wants.

Back in the day, etiquette had certain rules about invitations to events. If you realized you were not invited to something, you pretended this didn't bother you. You would never approach the host and ask why you weren't invited. Rude and awkward. By the same token, the host and other guests did not discuss the event in your presence to avoid rudeness and awkwardness.

Now we live in a time when it is OK to confront people, to get things off your chest, to be open, to assert your rights, to get closure. That leads people to ask why they weren't invited -- to weddings, to parties, to tennis teams.

It would be nice if folks could go back to just assuming that they cannot be invited to everything in life and accepting it.

If someone asks you to be on the team, you can tell her:
"I have a lot of strong ladies on the team, so I can't guarantee that you would get a chance to play any matches." If she still want to join your team under that circumstance, then let her sign up, since obviously it's not about the tennis for her - it's about her need to feel included.

If someone whines about not being invited, tell her:
"I have a lot of strong ladies on the team, and I didn't think it would be fair to you to invite you if it was unlikely that you'd get a chance to play a match."

The only way these exchanges might become awkward would be if the player had an overly inflated view of her own skill level.

What do guys do, anyway? If one guy has his buddies over to watch the game, do the ones who weren't invited ever learn about it or care?

I think guys tend to treat social occasions and competitive sports teams differently. If we didn't get invited to watch the football, yeah, we'd probably get a little hacked. The women seem to treat their sports teams as an extension of social occasion, from what I can tell.